Production of aliphatic acids



PRODUCTION OF ALIPHATIC ACIDS Alfred Frank Millidge, Coulsdon, Ian Kenneth Miles Robson, London, and Alec Elce, Epsom, England, as,- signors to The Distillers Company Limited, Edinburgh, Scotland, a British company Application April 13, 1954, Serial No. 422,932

Claims priority, application Great Britain April 25, 1953 4 Claims. (Cl. 260--533) In the liquid phase oxidation of lower paraflins containing from 4-8 carbon atoms, as described in the copending application Serial No. 385,272 filed October 9, 1953, the lower aliphatic acids of from 1-4 carbon atoms are obtained as the main product. There are formed, in addition, volatile neutral lay-products which may be separated from the acids and other less volatile products by distillation and may thus be oxidised further to yield C1C4 acids as described in the copending application Serial No. 385,182, filed October 9, 1953. There are also produced in the oxidation higher boiling substances, which may, for instance, be recovered by distilling oi the lower acids up to and including butyric acid. These residues comprise compounds containing acid, ketone, hydroxyl and ester groups, and are for the most part composed of poly functional compounds. The residues may also contain certain proportions of the lower aliphatic acids in the form of esters, or these may be present in minor amounts as the free acids owing to the difliculty of removing them completely by distillation.

We have now found that these residues can be oxidised in the liquid phase to provide further yields of the C1C4 aliphatic acids. The yields are, however, comparatively small, being of the order of 20% of the weight of residue charged, while a substantial quantity, amounting to 40% or more of the charge, is recovered as high boiling material. On removal of the more volatile products by distillation this high boiling material is obtained as a thick intractable pitch. In some cases, thils distillation has. even been accompanied by a violent exothermic reaction, believed to be a polymerisation, which gives rise to a residual hard coke.

We have also found that the residues from oxidation of parafiins may be combined with fresh paraflin feed and the mixture subjected to oxidation. In particular, it has been found that when oxidising parafiins in a continuous manner and separating the volatile products obtained, by distillation, from the higher boiling residues, these residues may be returned with the feed of fresh parafiin to the oxidation, and that when operating in this way the total amount of such residues in the system does not increase. This is a surprising observation since it would have been expected that a substantial proportion of said residues would have resisted oxidation and would therefore have accumulated in the system.

The invention accordingly comprises the novel processes and steps of processes, specific embodiments of which are described hereinafter by way of example and in accordance with which we now prefer to practice the invention.

Accordingly, the present invention is for a process for the production of aliphatic acids which comprises oxidising in the liquid phase a paraflin hydrocarbon of at least four carbon atoms with a gas consisting of or comprising molecular oxygen to produce lower aliphatic acids, distilling the oxidation product toseparate the volatile materials from the high boiling residues and thereafter oxidising in the liquid phase said residues with molecular oxygen to produce lower aliphatic acids. By the term niteci States Patent Patented July 23, 1957 volatile materials is meant the materials boiling below 170 C. at atmospheric pressure or its equivalent at other pressures, and by the term high boiling residues the ma terials boiling above this temperature. It is one embodiment of the invention to oxidise the residues under conditions such that they are substantially completely converted to volatile products, including the lower aliphatic acids. It is anoflier embodiment of our invention to oxidise these residues under conditions similar to those employed in the oxidation of the parafiins employed in our process, particularly by recycling the residues with the paratfins charged or fed to the oxidation.

It has been found that when the residues are recycled to the primary oxidation, the residues obtained by distillation of the product are fluid and easy to handle, and that on allowing these residues to cool succinic acid separates in crystalline form and may be recovered by simple filtration in a purity sufiicient for many purposes, and which by a single recrystallisation from a solvent, for example water, is obtained in a high state of purity. Accordingly in a preferred embodiment of our invention the said high boiling residues are recycled with the paraffins charged or fed to the oxidation, the residues from the distillation of the product being cooled to permit the separation of succinic acid prior to recycle to the oxidation.

The distillation of the oxidation product may be carried out in two or more columns as desired. Suitably the non? acidic volatile materials, for example boiling below 99 C. in the presence of water, are removed in a first column and the aqueous aliphatic acids in a second column.

When the residues are oxidised alone there may be employed as oxidising gas any gas containing molecular oxygen whether in the form of air or of mixtures poorer or richer in oxygen than is air, part of the molecular oxygen may, if desired, be in the form of ozone. The pressure used in the oxidation should be such as may be necessary to maintain the reactants substantially in the liquid phase, while the temperature should be sllfiiciently high to obtain an economically attractive rate of oxidation, suitably in the range ISO-250 C.

The oxidation of the residues may, if desired, be carried out in the presence of an oxidation catalyst. Where catalysts such as compounds of heavy metals have been used In the primary oxidation, the addition of oxidation catalysts to the residues is generally unnecessary, since these catalytic materials will be found in the distillation residues. The oxidation of the residues may be carried out batchwise or continuously and may be effected in any of the known type of apparatus in which the oxidising gas is intimately contacted with the liquid oxidation mixture.

When the residues are mixed with fresh paratfin feed and charged or fed to the primary oxidation the reaction conditions may be such as described in the said copending applications Serial Nos. 385,272 and 385,182, each filed October 9, 1953.

The hydrocarbon employed in the process of the present invention is preferably a parafiinic hydrocarbon fraction containing hydrocarbons of 4-8 carbon atoms, at least 40% by Weight of said fraction consisting of hydrocarbons of 6-8 carbon atoms, wherein at least 40% by weight of the parafiins of 6-8 carbon atoms of said fraction consists of paraflins having one or more methyl branch chains and wherein said fraction boils at a temperature not exceeding 100 C. Desirably the fraction boils Within the range about 15 to about C. By the expression methyl branch chain used above is meant a p'arafiin bearing a methyl group in a non-terminal posi tion in a chain.

Suitable feedstocks which may be used are straight run petroleum fractions containing a high proportion of branched molecules; the products from isomerisation of predominantly straight chain paraffins by aluminium chlo- Example 1 V A 15-95 C. parafiinic distillation fraction from crude petroleum, having an initial boiling point (ASTM) of 31.5" C. and a final boiling point (ASTM) of 96.5" C. having the composition:

Paraflins of Compo- Parafiins of 6-8 carbon sition, 6-8 carbon atoms having percent atoms, permethyl branch w./w. of cent w./w. of chains, perfraction traction cent w./w. of

traction n-Butane Ispentane n-Pentane- Cyclopentan 2: Z-Dimethylbu 2: 3-D imethylbuttme.

Benzene Methylcyclopentane" 2: 2-D imethylpentane. 2: 4Dimethylpentane. Cyclohexane 2:3Dimethy1pentane. Z-Methylhexane 1 3-D imethylcyclopentane (cis n-Heptane Methyl cyclohexane Toluene Unidentified material was oxidised and the product obtained was separated by distillation to yield as successive fractions volatile nonacidic products, lower aliphatic acids and water, and distillation residues. 1,356 grams of the residues were charged to a stainless steel reactor fitted with a stirrer, a bottom air inlet, a line for removing waste gas and means for measuring and controlling the temperature. The temperature of the reactor was raised to 160 C. and air introduced at a rate of about 3 litres/minute over a period of 15 hours during which the temperature was increased to 190 (3., and at the end of which time no further oxygen was absorbed. The products were subjected to distillation and there were recovered, for each 100 grams, residues charged, 2.3 grams formic acid, 9.2 grams acetic acid, 2.4 grams propionic acid and 0.59 gram butyric acid, as well as 38.1 grams of high boiling material, the remainder being water and gaseous products.

Example 2 The apparatus is shown diagrammatically in the accompanying figure and consists of a vertical tower 1 of 2.7 inches internal diameter and 6 litres capacity, having air feed points 2 at the base and about the midpoint. The tower is provided with oil jackets for controlling the temperature. Fresh hydrocarbon is introduced continuously by line 3 through a preheater 13. Both gaseous and liquid products are removed from the reactor by line 4 and after cooling in cooler 5 to a temperature below 80 C. are fed to a gas liquid separator 6 from which the waste gases are withdrawn by line 7. Small amounts of hydrocarbon in the waste gas may be recovered in part by suitable means, for example oil scrubbing, and recycled to the oxidation. The cooled liquid products are passed to a liquid-liquid separator 8 where the product separates into two liquid phases. The lower layer comprising the aqueous acid product is withdrawn at least in part through line 9. The upper layer comprising hydrocarbon, together with part of the lower layer, is returned to the reactor through the preheater 10. The withdrawn aqueous product is fed by line 9 through a preheater 15 to the mid-point of a packed distillation column 11 which has the equivalent of 5 theoretical plates above and below the feed point. The column is operated with a reflux ratio of 4:1, the temperature being about 66 C. at the head, approximately 82 C. at the feed point and 104105 C. in the reboiler. The distillate comprising the volatile non-acidic oxidation products are returned by line 12 for admixture with the feed to the reactor 1. The base product from this column is withdrawn by line 14 and fed to the mid-point of a second packed distillation column 15A which has the equivalent of 5 theoretical plates above and below the feed point. The column is operated with a reflux ratio of 3:1, the temperature being approximately 102-105" C. at the head, approximately C. at the feed point, and C. in the reboiler. The distillate withdrawn by line 16 comprises the aqueous aliphatic acid product. The base product from this still comprising the high boiling residues is withdrawn by line 17, cooled in the cooler 18, filtered in the filter 19 to separate the precipitated succinic acid which is removed, and the filtrate returned for admixture with the feed to the reactor 1.

The conditions of operation were as follows:

Temperature C.) Pressure (lbs/sq. inch) 600 Waste gas rate (l./min.average) 28.5

Hydrocarbon consumption rate (g./hour) (av.) 475 Volatile non-acidic products recycle rate (g./hour) 368 C1-C4 acids and water production rate 533 Liquid residues recycle rate (g./hour) 126 The overall yields of C1C4 acids and succinic acid on a no-loss basis, but without allowing for material removed in the waste gas, were as follows:

Grams per 100 grams hydrocarbon used up The oxidation of the residues was virtually complete, the overall production of residues being 1.6 grams per 100 grams of hydrocarbon consumed, and this figure was accounted for almost entirely by the hold up of partially oxidised material in the reactor.

We claim:

1. A continuous process for the production of aliphatic acids which comprises oxidising in the liquid phase a paraffin hydrocarbon of four to eight carbon atoms with molecular oxygen to produce lower aliphatic acids, separating at least part of the oxidation product containing aliphatic acids of one to four carbon atoms and a high-boiling residue having a boiling point of at least 170 C., distilling said separated oxidation product to separate and recover said acids from the high-boiling residues, returning substantially all of the resulting residues and admixing same with the supply of parafiin hydro carbon to the oxidation zone, and oxidising said residues in the liquid phase to produce further aliphatic acids of one to four carbon atoms.

2. A continuous process for the production of aliphatic acids as claimed in claim 1 wherein the separated oxidation product is cooled to a temperature below 80 and allowed to separate into two layers, the upper layer returned to the oxidation zone and at least part of the lower layer subjected to the said distillation.

3. A continuous process for the production of aliphatic acids as claimed in claim 1 wherein the oxidation is carried out at a temperature in the range ISO-250 C.

4. A continuous process for the production of aliphatic acids, which comprises feeding a parafiinic hydrocarbon fraction containing hydrocarbons of four to eight carbon atoms, at least 40% by weight of said fraction consisting of hydrocarbons of siX to eight carbon atoms wherein at least 40% by weight of the parafiins of six to eight carbon atoms of said fraction consists of paraflins having at least one methyl branch chain and boiling within the range about 15-95 C., and oxidising same in the liquid phase to produce aliphatic acids of one to four carbon atoms and a high-boiling residue having a boiling point of at least 170 C., separating said aliphatic acids and returning said residue to said paraffinic hydrocarbon fraction for further oxidation therewith to produce further aliphatic acids of one to four carbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS 2,241,487 Slotterbeck May 13, 1941 2,265,948 Loder Dec. 9, 1941 2,530,512 Drewitt Nov. 21, 1950 2,578,306 Hull Dec. 11, 1951 OTHER REFERENCES Perry: abstract of application Serial No. 121,229, 652 O. G. 892, November 20, 1951. 

1. A CONTINUOUS PROCESS FOR THE PRODUCTION OF ALIPHATIC ACIDS WHICH COMPRISES OXIDISING IN THE LIQUID PHASE A PARAFFIN HYDROCARBON OF FOUR TO EIGHT CARBON ATOMS WITH MOLECULAR OXYGEN TO PRODUCE LOWER ALIPHATIC ACIDS, SEPARATING AT LEAST PART OF THE OXIDATION PRODUCT CONTAINING ALIPHATIC ACIDS OF ONE TO FOUR CARBON ATOMS AND A HIGH-BOILING RESIDUE HAVING A BOILING POINT OF AT LEAST 170*C., DISTILLING SAID SEPARATED OXIDATION PRODUCT TO SEPARATE AND RECOVER SAID ACIDS FROM THE HIGH-BOILING RESIDUES, RETURNING SUBSTANTIALLY ALL OF THE RESULTING RESIDUES AND ADMIXING SAME WITH THE SUPPLY OF PARAFFIN HYDROCARBON TO THE OXIDATION ZONE,AND OXIDISING SAID RESIDUES IN THE LIQUID PHASE TO PRODUCE FURTHER ALIPHATIC ACIDS OF ONE TO FOUR CARBON ATOMS. 